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Raised in an orphanage after witnessing his sister's suicide, Satsuki Kakeru's life has already been dark and cruel enough. When he and his friend Yuka are thrown into a alternate dimension called Red Night, it takes a gruesome turn from bad to horrific. Now, trapped flickering between our world and the Red, Kakeru quickly discovers that he and Yuka are not alone, and that, together with the other so-called fragments, they must defeat the malevolent entities known as the Black Knights.

Facing monsters from beyond imagination and at the center of a deadly hunt, Kakeru's only chance of saving their lives now hinges on finding the power buried within himself and unraveling the mystery of his own origins and family... if he can survive long enough to do it! The horrors of the Red Night are just the beginning of the nightmare as shocking truths are exhumed! When the Sky turns Red, the Moon turns Black, and monsters begin roaming the streets, Satsuki Kakeru is at a loss for what to do. Along with his best friend Yuka, they try to decipher why they have been sent to this strange world, which is seemingly empty aside from themselves.However, when the "Red Night" ends, Kakeru and Yuka believed it was all a dream, until it happens again and they are left in a dangerous situation. They meet four others in the same predicament: Kusakabe Misuzu, an expert swordswoman, Tachibana Kukuri, a strange mute girl who looks uncannily like Kakeru's deceased sister, Hirohara Yukiko, a lively young girl whose personality reverts to that of a cold killer when her glasses are removed, and Tajima Takahisa, a young pyrokineticist.As the six of them band together to survive and discover what this mysterious world is, things take a turn for the worse as six shadows appear before them... Seven years ago, Kakeru’s sister killed herself, leaving him alone. Now a teenager, Kakeru lives a quiet life with his friend Yuka and other classmates – but things change forever when he and Yuka are mysteriously transported to a frightening parallel world filled with monsters and a deep crimson sky. Though they manage to escape, Kakeru soon discovers that they aren’t alone; several other students at school also travel to the other dimension, and armed with powerful weapons they battle against its dark inhabitants. Why must they fight, and what is their purpose in the Red Night?